1. Field of the Invention
The present invention relates to torque limiters which are capable of adjusting the magnitude of the transmitted torque, and more particularly to such torque limiters which utilize magnet coupling to transmit the torque.
2. Description of the Prior Art
Torque limiters are used to transmit torque which does not exceed a predetermined magnitude. FIG. 1 shows a conventional torque limiter utilizing permanent coupling to transmit torque, which is disclosed in Japanese laid-open utility model application No. 58-187627. The disclosed torque limiter comprises an interior and an exterior rotating portion, the torque being transmitted from the exterior to the interior portion, or vice versa.
The interior portion of the torque limiter of FIG. 1 is constituted as follows. The interior rotating member 1 is made of a non-magnetic material and has a main hollow cylindrical portion la fixedly mounted by a key on the shaft 2 of an apparatus to or from which the torque is transmitted. The member 1 has a annular flange 1b projecting radially from the main cylindrical portion 1a thereof. Annular disk 1c of magnetic material is fixedly secured to the flange 1b by means of a shrinkage fit to form a rotating disk therewith. Thus, in the case of the torque limiter of FIG. 1, the rotating disk is made of two pieces, i.e., of an annular disk 1c of magnetic material and a non-magnetic flange 1b of the rotating member 1. However, the disk 1c and the member 1 may be made of a single piece of magnetic material.
The structure of the exterior rotating portion of the torque limiter of FIG. 1 is as follows. A pair of exterior rotating members 3 and 4 are rotatably supported on the interior rotating member 1 by the bearings 3a and 4a, respectively. The exterior rotating members 3 and 4 have an inner axially directed surfaces 3b and 4b opposing the axially directed surfaces of the interior rotating disk 1c and 1b. A ring 5 of non-magnetic material is fitted into the recess formed by the opposing inner step-formed portions 3c and 4c formed on the outer circumferential surfaces of the exterior rotating members 3 and 4. Further, an annular permanent magnet 6 is fitted around the ring 5 between the outer step-form portions 3d and 4d of the exterior rotating members 3 and 4. The exterior rotating member 3 has an outer diameter slightly greater than that of the permanent magnet 6, and has a male thread 3e formed on the outer circumferential surface thereof, around which a hollow cylindrical adjustment member 7 is fitted by means of a female thread 7a formed on the inner circumferential surface thereof and screwed around the male thread 3d of the member 3. The adjustment member 7 is turned around the member 3 to be moved in the axial direction thereof. Thus, the clearance gap g between the inner side surface of the flange 4c of the exterior rotating member 4 and the end surface of the adjustment member 7 opposing thereto can be adjusted by the turning of the member 7, thereby varying the magnetic resistance of the magnetic circuit .phi.1. The screw 8 is turned in to fasten the adjustment member 7 to the exterior rotating member 3. Sealing members 3f and 4f, such as mechanical seals, and the exterior rotating members 3 and 4, are disposed between the inner rotating member 1 and the exterior rotating members 3 and 4, respectively.
A predetermined amount 9 of a powder of a magnetic material fills the gaps formed between the opposing surfaces of the rotating disk 1c and the exterior rotating members 3 and 4. Thus, the powder 9 forms part of the magnetic circuit .phi.2 together with the exterior rotating members 3 and 4 and the permanent magnet 6, and is magnetized by the magnetic flux passing therethrough, to bond the disk 1c with the exterior rotating members 3 and 4.
When the load or the resistive torque on the interior rotating member 1 does not exceed a pre-set value, the interior rotating disk 1c and the pair of exterior rotating members 3 and 4 are bonded together by the magnetized powder 9 and are rotated together without slip, transmitting the torque from the input to the output portion of the torque limiter. When, however, the resistive torque on the interior rotating member 1 exceeds a pre-set value and overcomes the coupling force of the magnetized powder 9, the disk 1c slips with respect to the exterior rotating members 3 and 4, allowing relative rotation thereof with respect to the exterior rotating members 3 and 4, thereby limiting the transmitted torque to the pre-set value.
The maximum value of the transmitted torque is adjusted as will be explained hereunder.
The magnetic circuits .phi.1 and .phi.2 passing through the permanent magnet 6 are formed. The first circuit .phi.1 passes through the permanent magnet 6, the gap g, and the member 7, as well as portions of the exterior rotating members 3 and 4. The second circuit 02 passes through the permanent magnet 6, portions of the exterior rotating members 3 and 4, the interior rotating disk 1c disposed therebetween, and the powder of magnetic material 9. By varying the dimension of the gap g between the members 4 and 7, the magnetic reluctance of the first magnetic circuit .phi.1 and the magnetic flux passing therethrough are changed. Thus, the amount of magnetic flux passing through the second magnetic circuit .phi.2 is varied, and the value of the transmitted torque is changed.
The type of the permanent magnet torque limiters as described above, however, have disadvantages as will be explained hereunder.
In the adjustment of the maximum value of the transmitted torque, only the magnetic reluctance of the first or leakage magnetic circuit .phi.1 passing through the gap g is changed, while that of the second circuit .phi.2 passing through the interior rotating disk 1c and the powder 9 remains constant. Thus, it is not possible to limit the transmitted torque to a very small value near zero, because even when the gap g is closed, there remains a substantial amount of magnetic flux in the second magnetic circuit .phi.2. Further, the total amount of the magnetic flux passing through the first and second magnetic circuits .phi.1 and .phi.2, i.e., the amount of flux passing through the permanent magnet 6, changes with the adjustment of the gap g. Thus, demagnetization of the permanent magnet is incurred by repeated adjustment operations over a long period of usage. Furthermore, the permanent magnet 6 incorporated in the torque limiter should have a linear demagnetization curve, and accordingly the permanent magnet 6 which can be used therein is limited to magnets having such a property.